Concrete casting techniques may utilize spaced forms held in position by supports movable up a wall under construction. Concrete is poured between the forms and on top of earlier formed sections of the wall. Examples of construction using such techniques include cooling towers and smokestacks. Prior art jacking systems are primarily concerned with ascending, where the movable rig is raised until the structure is constructed, and then the rig is disassembled for removal.
In general, a dual pawl system may raise a carriage along a vertical support. Two pawls alternate supporting the weight of the carriage by engaging against a series of protruding or recessed lugs evenly spaced upon the vertical support. As a force is applied to the carriage, the weight of the carriage is alternately borne by one pawl as the other pawl is relocated to a new lug located immediately above the previous lug as the carriage is raised.
An example of a prior art ascending jacking system is shown by FIG. 1A. A platform bearing carriage 100 ascends a vertical support 170 as concrete forms are poured and installed. FIG. 1B shows a detail of the jacking system of the prior art carriage 100. Generally, a lower pawl assembly 130 and an upper pawl assembly 120 alternately support the weight of the carriage 100 upon a plurality of lugs 175 spaced along the vertical support 170 as the carriage ascends. A hydraulic jack 110 extends and retracts a plunger 115 attached to the upper pawl assembly 120. As the plunger extends, the upper pawl assembly 120 is engaged with a lug 175, bearing the weight of the carriage 100 as weight is relieved from the lower pawl assembly 130, allowing the lower pawl assembly to ascend. The spring loaded lower pawl assembly 130 retracts slightly as it passes over a lug 175, and then is forced against the vertical support 170 as the lower pawl assembly 130 clears the lug 175. Similarly, as the hydraulic jack 110 retracts the plunger 115, the lower pawl assembly 130 supports the carriage 100 as the upper pawl assembly 120 is raised.
The prior art carriage 100 is only configured to ascend the vertical support 170. A typical concrete pouring system may incorporate multiple carriages to simultaneously support and raise a number of platforms. However, ascending only jacks are not generally practical for maintenance, repair, or rebuilding of structures, for example, smokestacks and cooling towers, because they cannot easily be moved to lower positions. While some prior art jacking systems are capable of descending, they do not do so easily or automatically. In particular, descent may require manual releasing of individual supports, but it is generally impractical to release multiple supports simultaneously.
Therefore, there is a demonstrated need in the industry for a jacking system that addresses some or all of these deficiencies.